Overcoming T-cell exhaustion: New therapeutic targets in HCC immunotherapy
Authors: Takahiro Seimiya, Motoyuki Otsuka, Mitsuhiro Fujishiro
Journal: Hepatology (2023)

HCC is the third leading cause of cancer-related deaths worldwide, accounting for ∼800,000 deaths annually. The treatment of HCC depends on liver function and the extent of tumor progression, with systemic chemotherapy being the treatment of choice for advanced HCC. Systemic chemotherapy for HCC has advanced dramatically over the last decade, and immune checkpoint inhibitors are now the first-line systemic therapy of choice for advanced HCC. In situations of prolonged exposure to antigens, such as chronic viral infection or cancer, T-cell activity is attenuated. This loss of T-cell immune cell activity is known as T-cell exhaustion and is characterized by the expression of various inhibitory receptor proteins, such as programmed cell death protein 1 and T-cell Ig and mucin domain-containing-3, which are regulated by the transcription factor thymocyte selection-associated high mobility group box.1 These inhibitory receptor proteins function as "checkpoints," suppressing T-cell effector function and proliferation. Immune checkpoint inhibitors block these inhibitory receptor signals and prevent T-cell exhaustion, resulting in tumor suppression. In a representative clinical trial for advanced HCC, combination therapy with the programmed cell death protein 1 inhibitor atezolizumab and vascular endothelial growth factor inhibitor bevacizumab significantly prolonged overall survival and progression-free survival compared with lenvatinib, a multikinase inhibitor that had been the standard of care up to that point.2 Although immune checkpoint inhibitors for HCC are a breakthrough, <30% of patients achieve a complete or partial response. To improve the efficacy of immune checkpoint inhibitors, it is necessary to understand the molecular mechanisms underlying T-cell exhaustion. Wang et al.3 demonstrated that the dedicator of cytokinesis 2 (DOCK2)-cholesterol sulfate (CS)-sulfotransferase 2B1 (SULT2B1) axis plays an important role in T-cell exhaustion in the tumor microenvironment of HCC. They first reanalyzed previously published HCC proteomics, single-cell RNA sequencing, and TCGA data and found that the number of tumor-infiltrating CD8+ T cells correlated with the expression level of DOCK2 in CD8+ T cells. DOCK2 is a Rac-specific guanine nucleotide exchange factor that is found primarily in hematopoietic cells and is essential for leukocyte migration. To elucidate the role of DOCK2 in the pathogenesis of HCC, Wang et al. investigated the effect of CPYPP (a small-molecule inhibitor of DOCK2) using an orthotopic HCC transplantation model. CPYPP administration reduced T-cell infiltration, increased T-cell exhaustion, and shortened mouse survival. In addition, CD8+ T-cell–specific DOCK2 knockout mice also showed a similar increase in exhausted T cells and tumor progression. These results indicate that impaired function of CD8+ T cells leads to impairment of anti-tumor immunity. To elucidate the mechanism by which DOCK2 inhibition induces T-cell exhaustion, immunoprecipitation, and mass spectrometry analysis were carried out, and it was found that DOCK2 interacts with the transferrin receptor (TFRC), and its inactivation promoted TFRC degradation, impaired mitochondrial function, and induced T-cell exhaustion. Mechanistically, DOCK2 binds to TFRC via the DHR2 domain, a domain with a conserved GTP Exchange Factor (GEF) function in the DOCK superfamily. CPYPP inactivates DOCK2 by occupying the active pocket of the DHR2 domain, thereby blocking its interaction with TFRC. The dissociation of TFRC from DOCK2 promotes its ubiquitination and degradation. Although the exact mechanism by which TFRC degradation causes mitochondrial dysfunction remains unknown, this study showed that DOCK2 inactivation and TFRC degradation cause mitochondrial dysfunction and T-cell exhaustion. To evade the host immune surveillance, cancer cells induce various forms of immunosuppression in the tumor microenvironment. When CD8+ T cells were incubated with HCC cell culture supernatants, DOCK2 activity was inhibited in CD8+ T cells. This suggests that HCC cells may evade host immunity by inducing T-cell exhaustion via DOCK2 inhibition. A metabolomic analysis of the HCC culture supernatants revealed that CS was upregulated. CS is a natural DOCK2 inhibitor that contributes to the immunosuppressive microenvironment of the eye.4 In the NRAS/c-MYC plasmid hydrodynamic tail vein injection model, CS inhibited DOCK2 activity, induced T-cell exhaustion, and promoted tumor progression. Furthermore, knockout of SULT2B1, an enzyme essential for CS production, alleviates T-cell exhaustion and inhibits HCC progression. Consistent with in vivo experimental results, high SULT2B1 expression was correlated with poor survival in TCGA dataset. These results indicate that the SULT2B1-CS-DOCK2 axis is crucial for HCC progression. Wang et al. analyzed publicly available toxicotranscriptomic data and performed drug screening using a small-molecule library to identify the drugs that can regulate SULTB1 and identified acetaminophen and tolazamide as potential candidates. Acetaminophen, a commonly used antipyretic analgesic, attenuated the effect of HCC immunotherapy by promoting SULT2B1 transcription. The fact that it exhibited tumor-promoting effects, at least in the model used in this study, suggests that drugs used in combination with immunotherapy should be administered with caution. Tolazamide, an oral blood glucose-lowering drug, was also found to enhance the efficacy of HCC immunotherapy by inhibiting SULT2B1 activity. Although it has been suggested that tolazamide may enhance the efficacy of immunotherapy, several issues need to be resolved before these findings can be applied clinically. Due to their impaired liver function, there is concern that HCC patients may be more prone to the hypoglycemic side effects of tolazamide. In addition, because the DOCK2-CS-SULT2B1 axis is important for protecting the eye against UV-induced photokeratitis and antigen-induced conjunctivitis by maintaining an immunosuppressive state, there is concern that systemic blockade of this pathway could result in severe eye inflammation. Although this study identified candidate drugs that may modulate the SULT2B1-CS-DOCK2 axis, clinical trials are required to confirm the additive effect and safety of the combined use of candidate drugs and immunotherapy in HCC. Exhausted T cells are a heterogeneous population, with various epigenetic, transcriptional, and metabolic mechanisms thought to be involved in the transition from progenitors to terminally exhausted T cells.5 Wang et al. suggested that the SULT2B1-CS-DOCK2 axis is a potential therapeutic target for overcoming T-cell exhaustion and improving the efficacy of HCC immunotherapy. Tatsuguchi et al.6 reported comparable results using breast cancer cells. However, the exact mechanism by which the SULT2B1-CS-DOCK2 axis enhances the efficacy of immunotherapy remains to be elucidated. Enhanced infiltration of CD8 T cells into the tumor microenvironment and enhanced effector function of CD8 T cells are 2 possible explanations derived from this study. In the CD8-specific DOCK2 knockout model, the fraction of CD8 T cells within tumor-infiltrating lymphocytes was reduced. In contrast, the SULT2B1 inhibitor promoted the infiltration of CD8 T cells. These results suggest that restoring DOCK2 function may promote the infiltration of CD8 T cells into the tumor. Another possibility is to enhance the effector function of CD8 T cells. SULT2B1 inhibitors decreased the expression of inhibitory receptors on CD8 T cells, suggesting that this may have either delayed T-cell exhaustion or reprogrammed exhausted T cells into progenitor cells. If T-cell exhaustion is delayed, then treatment with SULT2B1 inhibitors must be initiated before the irreversible advancement of the T-cell exhaustion program. Although many questions remain unanswered, elucidating the detailed mechanisms by which the SULT2B1-CS-DOCK2 axis regulates T-cell exhaustion in HCC could lead to the development of effective immunotherapy for HCC (Figure 1).FIGURE 1: CS secreted by HCC cells inactivates the DOCK2 and induces T-cell exhaustion. Tolazamide inhibits CS production by blocking , thereby suppressing DOCK2 inactivation in T cells. Tolazamide in combination with immunotherapy enhances therapeutic efficacy. Abbreviations: CS, cholesterol sulfate; DOCK2, dedicator of cytokinesis 2; PD-1, programmed cell death protein 1; SULT2B1, sulfotransferase 2B1.